Digital image capture technology is rapidly replacing film based technology. Both digital still and video cameras are now widely available. Also available are photo-video camera products that combine digital still and video capabilities into A single device. Digital still photography requires an image sensor with as high as possible a resolution, for example a 5 Mega-Pixel (MP) sensor in an approximately 2600×2000 pixel format. The high resolution is required to match the image quality of film based photography. Digital video photography, on the other hand, only requires a much lower resolution, typically about VGA resolution of 640×480 pixels at 30 frames per second. Consequently, for photo-video cameras the video images are typically sampled at an appropriate sample rate with respect to the native resolution of the sensor. That is to say that although an image is formed using the entire sensor resolution of 2600×2000 pixels only a reduced number of those pixels is used to form the video image. Typically, the sampling process also involves some element of binning where the electrical charge from a block of pixels is combined before analogue to digital conversion occurs.
This inherent mismatch between still and video resolution offers the opportunity to provide some digital zoom facility. Taking the previous example of a 5MP sensor, there is a possible four fold difference in magnification between VGA resolution images obtained on the one hand by sampling every fourth pixel horizontally and vertically over the whole sensor and on the other hand by sampling a 640×480 pixel region from the centre of the sensor at the sensor's native resolution. Digital zoom is therefore an inherently discrete process, in contrast to the zoom effect provided by an optical zoom lens, which is virtually continuous. It is known to provide digital still and video cameras that offer some form of digital zoom as either an alternative to optical zoom or as a way to increase the overall zoom factor. In these schemes, the digital zoom process produces a cropped image that is generally up-sampled back to the full still/video resolution using some form of pixel interpolation.
Interpolation, sometimes referred to as resampling, is an imaging method for increasing the size of a digital image. Various interpolation schemes are known. For example, the simplest way to increase the size of a digital image is called “nearest neighbour” interpolation. This simply takes the colour of the new pixel's nearest neighbour and uses it for this new pixel. However, this results in visible pixelisation and visible jaggies. A more sophisticated interpolation scheme is bilinear interpolation, which is commonly used in digital cameras. This produces relatively smooth edges with hardly any jaggies. An output pixel is determined as a bilinear function of the four pixels in the nearest 2×2 neighbourhood of the pixel in the source image. Bicubic interpolation is an even more sophisticated scheme and produces very smooth edges with hardly any jaggies. An output pixel is a bicubic function of the 16 pixels in the nearest 4×4 neighbourhood of the pixel in the source image. This is a commonly used interpolation method in photographic editing software. However, all interpolation involves creating image data that has not been actually recorded and the result is therefore always inferior to an actual image captured by a true optical zoom.
As mentioned above, it is known to provide digital still and video cameras that provide both digital and optical zoom facilities. For example, U.S. Pat. No. 5,684,532 describes a video camera system for use with a security surveillance system in which a digital zoom facility is provided to allow rapid changes in image magnification, with an optical zoom facility provided as well. Having switched to the desired magnification factor using the digital zoom facility, the optical zoom lens is operated to match the magnification factor provided by the digital zoom. As the optical zoom magnification factor increases the electronic zoom factor is correspondingly decreased so that the overall magnification factor of the image is substantially unchanged. A similar scheme is also described in the U.S. Pat. No. 5,701,157. In this patent a zoom apparatus is described that uses a digital zoom facility simultaneously with an optical zoom facility to compensate for the non-linear magnification factor of the optical zoom facility. A similar system is also described in U.S. Pat. No. 5,420,632.